Phil Zullo, a Toronto resident, told Canadian Press he saw police arresting a man who had been driving a Ryder rental truck, which Ryder Systems Inc has confirmed.

There were people "strewn all over the road," Mr Zullo said.

"I must have seen about five, six people being resuscitated by bystanders and by ambulance drivers," Mr Zullo said.

"It was awful. Brutal."

The incident occurred as cabinet ministers from major industrial countries gathered in Canada to discuss a range of international issues in the run-up to the G7 meeting near Quebec City in June.

While police did not immediately say whether the incident was deliberate, Reuters said at least one witness revealed the driver appeared to be deliberately targeting people as he drove for up to one mile on the busy sidewalk.

Toronto police spokeswoman Meaghan Gray said: "At this point it's too early to tell what, if any, motive there was."

CNN reported that the driver in the van incident was previously known to Toronto authorities, according to a US law enforcement official.

As emergency responders rushed to deal with the incident - which occurred as office workers were heading out to lunch - officials at the city's Sunnybrook hospital said it was treating five people who were described as being in a "critical condition", with two others whose condition was said to be "serious".

The hospital received a total of 10 people from the van incident.

"Unfortunately, two patients arrived vital signs absent and were pronounced dead upon arrival," the hospital said in a statement.

At least two patients were being treated at St Michael's Hospital, according to the Toronto hospital.

Police shut down the Yonge and Finch junction following the incident and Toronto's transit agency said it had suspended service on the subway line running through the area.

Prime minister Justin Trudeau expressed his sympathies for those involved.

"Our hearts go out to everyone affected," he said in Ottawa. "We are going to have more to learn and more to say in the coming hours."

Toronto Mayor John Tory said that "this is a time when the community should come together", adding: "My thoughts are with those affected by this incident and the frontline responders who are working to help those injured.

"I have offered any and all assistance that the city can provide to help this investigation."

He concluded: "I hope that we will as a city remind ourselves of the fact that we are admired around the world for being inclusive and for being accepting and understanding and considerate and that we are united in standing in solidarity with those who have fallen victim to this terrible tragedy today."